Systems and methods for digital upconversion for television signals

ABSTRACT

Systems and methods for digital upconversion of baseband television signals that can be used in cable television headend systems are provided. In one embodiment, the system includes a digital frequency adjustment system and a digital to analog conversion system. In a feature of the embodiment, the digital frequency adjustment system consists of set of digital upconversion and upsample elements that shift upwards the frequency of baseband signals. In a further feature of the embodiment, a tree structure of sets of upsample and upconversion elements is used. In another embodiment, the system includes digital and analog frequency adjustment systems in which the frequencies of the input signals are partially upshifted within both the digital and analog domains. Methods for digital upconversion of television signals are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/452,211, filed Jun. 3, 2003, which issued as U.S. Pat. No. 6,724,335on Apr. 20, 2004, and which is hereby incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cable television, and moreparticularly, to cable television signal transmission.

2. Background of the Invention

Cable television systems generally require a conversion system forfrequency converting the transmitted channels from baseband frequenciesto their designated RF frequencies for transmission over the cablemedium. This system is typically part of a cable television (CATV)headend system, where the composite, multi-channel CATV signal isgenerated and amplified for distribution to customers.

Within existing headend systems, each frequency converters typically usemultiple analog mixing stages, with one or more analog phase-lockedloops (PLL) to generate the local oscillators. One converter is requiredfor each channel, and there can be more than one hundred channels in atypical CATV system. These systems are often costly and requiresignificant amounts of hardware. Additionally within existing systems,control of signal amplitude for each channel can be complex.

What is needed is a cost-effective system and method for frequencyconverting baseband television signals and creating composite,multi-channel CATV signals within a CATV headend system.

SUMMARY OF THE INVENTION

The invention is directed to systems and methods for digitalupconversion of baseband television signals and other types of signals,such as those associated with cable modems, that can be used in cabletelevision headend systems. In one embodiment, the digital headendupconversion system includes a demultiplexer, a digital frequencyadjustment system and a digital to analog (DAC) conversion system. Inone embodiment the digital frequency adjustment system includes a set ofupsample and upconversion elements that shift upwards the frequency ofbaseband signals. In another embodiment, a tree structure of sets ofupsample and upconversion elements is used. The digital to analogconversion system includes a single digital to analog converter or a setof converters.

An alternative embodiment of the digital headend upconversion system isa digital hybrid headend upconversion system that includes ademultiplexer, a digital frequency adjustment system and an analogfrequency adjustment system. In this embodiment, the frequencies ofbaseband signals that are input to the upconversion system are partiallyupshifted within the digital domain and partially upshifted within theanalog domain. The digital frequency adjustment system is as describedabove, except that the frequencies of the baseband signals are partiallyadjusted rather than upshifted to final desired frequencies fordistribution. The analog frequency adjustment system includes a set ofdigital to analog converters followed by a set of band pass filters,followed by a set of mixers, followed by another set of band passfilters, followed by another set of mixers, and finally followed by aset of low pass filters. The outputs of each of the low pass filters aresummed together to form the desired frequency upconverted compositesignal for distribution throughout a cable network. In a furtherfeature, within a digital or digital hybrid upconversion system, anindividual channel gain adjustment system can be included to allowprecise gain adjustment controls for individual channels.

Methods for digital upconversion of television signals are alsoprovided. In one embodiment, the method includes receiving digitalbaseband television signals, demuxing those signals, upsampling andupconverting the demuxed signals, then recombining the signals andperforming a digital to analog conversion. In one embodiment, upsamplingand upconverting the demuxed signals occurs in a two steps. Inalternative embodiments, a tree structure of upsampling and upconversionelements in used, such that upsampling and upconverting occurs inmultiple two-step phases.

In another embodiment, the frequencies of digital input signals arepartially upshifted within the digital domain and partially upshiftedwithin the analog domain.

Use of the invention provides two principal benefits. First, use of theinvention reduces the cost and complexity of hardware needed for a cabletelevision headend system. Second, use of the invention simplifiesdigital control of channel amplitude for the television signals.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments of theinvention are described in detail below with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described with reference to the accompanying drawings.In the drawings, like reference numbers indicate identical orfunctionally similar elements. The drawing in which an element firstappears is indicated by the left-most digit in the correspondingreference number.

FIG. 1 is a diagram of a digital headend upconversion system, accordingto an embodiment of the invention.

FIG. 2 is a diagram of a digital frequency adjustment system, accordingto an embodiment of the invention.

FIG. 3 is a diagram of a digital frequency adjustment system thatincludes cascading upsample and upconversion elements, according to anembodiment of the invention.

FIG. 4A is a diagram of a digital to analog converter system, accordingto an embodiment of the invention.

FIG. 4B is a diagram of a digital to analog converter system thatincludes a series of digital to analog converters, according to anembodiment of the invention.

FIG. 5A is a diagram of a digital hybrid headend upconversion system,according an embodiment of the invention.

FIG. 5B is a diagram of an analog frequency adjustment system, accordingto an embodiment of the invention.

FIG. 6 is a diagram of an upconversion element, according to anembodiment of the invention.

FIG. 7 is a method for digital upconversion of baseband televisionsignals, according to an embodiment of the invention.

FIG. 8 is a method for digital hybrid upconversion of basebandtelevision signals, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those skilled inthe art with access to the teachings provided herein will recognizeadditional modifications, applications, and embodiments within the scopethereof and additional fields in which the invention would be ofsignificant utility.

FIG. 1 illustrates digital headend upconversion system 100, according toan embodiment of the invention. Digital headend upconversion system 100includes demultiplexer 110, digital frequency adjustment system 120 anddigital to analog converter (DAC) system 130. An input signal isprovided to digital headend upconversion system 100 over connection 135,and an output signal is transmitted from digital headend upconversionsystem over connection 145. Digital headend upconversion system 100 canbe used within a cable television headend system. When used within acable television headend system, inputs into demultiplexer 110 aremultiple baseband television channels in a digital format. The outputsof demultiplexer 110 are coupled to the input of digital frequencyadjustment system 120, and the outputs of digital frequency adjustmentsystem 120 are coupled to the input of DAC system 130, which transmitsits output over connection 145.

In an alternate embodiment of digital headend upconversion system 100,demultiplexed signals are provided to the system, so that demultiplexer110 is not required. In another alternate embodiment of digital headendupconversion system 100, a individual channel gain adjustment system canbe coupled to either the output of demultiplexer 110 or to the output ofdigital frequency adjustment system 120. Use of an individual channelgain adjustment system allows individual channel gains to be scaleddigitally which is more precise and less prone to drift than currentanalog approaches.

Digital headend upconversion system 100 converts digital basebandtelevision signals to an analog signal in which the digital basebandtelevision signals have been upconverted in frequency to the desiredradio frequency (RF) frequencies to create a multi-channel RF spectrum.This output, or multi-channel RF spectrum, can then be distributed overa cable television distribution system to individual cable subscribers.

Digital frequency adjustment system 120 can be implemented in a numberof alternative embodiments. FIG. 2 is a diagram of digital frequencyadjustment system 120, according to one embodiment of the invention. Inthis embodiment, digital frequency adjustment system 120 includesupsample elements 205A, B, C, and n; upconversion elements 210A, B, C,and n; and a summing device 220.

As discussed with respect to FIG. 1, digital baseband television signalscan be input to demultiplexer 110. Demultiplexer 110 provides a set ofoutput signals to an array of upsample elements 205A, 205B, 205C through205 n, such that each of the baseband signals outputted fromdemultiplexer 110 is transmitted to an upsample element that willupsample the baseband signal. The number of upsample elements 205 usedwill be a function of the number of baseband channels to be upconverted.The upsample elements interpolate intermediate data points betweensignal points, and add those to the signal to facilitate less complexdigital to analog conversion.

Outputs from the upsample elements 205A, B, C and n are coupled to theinputs of upconversion elements 210A, B, C or n. As discussed below withrespect to FIG. 6, the design of each of upconversion elements 210 isthe same, except for their operating frequencies. Each upconversionelement 210 will be coupled to one upsample element 205. For example,upsample element 205A is coupled to upconversion element 210A, upsampleelement 205B is coupled to upconversion element 210B, upsample element205C is coupled to upconversion element 210C and so forth, such thatupsample element 205 n is coupled to upconversion element 210 n. Theoutputs from all of the upconversion elements 210 are coupled to summingdevice 220. Summing device 220 combines these signals to output anupconverted digital signal. The output of summing device 220 is thencoupled to the input of DAC system 130. As can be observed in FIG. 1,the signals traversing digital frequency adjustment system 120, remainin digital form and therefore enable significant digital control ofchannel amplitudes.

FIG. 3 provides an alternative embodiment of digital frequencyadjustment system 120, according to an embodiment of the invention. Inthis embodiment, the upsampling and upconverting is carried out in atree structure or series of upsampling and upconverting steps tominimize component complexity. In this case, digital frequencyadjustment system 120 includes a first set of upsample elements 305A,305D, 305X and 305 n; a first set of upconversion elements 310A, 310D,310X, and 310 n; a first set of summing devices 315A and 315B; a secondset of upsample elements 320A and 320B; and a second set of upconversionelements 330A and 330B; and a summing device 325. As in the embodimentdepicted in FIG. 2, the number of upsample elements within the first setof upsample elements will be a factor of the number of basebandchannels, and the number of steps within the tree structure.

As in the previous case, demultiplexer 110 provides a set of outputsignals to an array of upsample elements 305A, 305D, 305X through 305 n,such that each of the baseband signals output from demultiplexer 110 istransmitted to an upsample element. Outputs from upsample elements 305A,D, X and n are coupled to the inputs of upconversion elements 310A, D, Xand n that will upconvert the baseband signal to a desired RF frequency.Each upconversion element 310 will be coupled to one upsample element305.

Up until this point, the embodiment described with respect to FIG. 3appears the same as the embodiment described with respect to FIG. 2. Atthis point, however, the embodiments differ. Rather than having theoutputs from the upconversion elements coupled to a single summingdevice, as was the case with respect to the embodiment depicted in FIG.2, the outputs from the upsample elements are coupled to two summingdevices. More precisely, the outputs of upconversion element 310Athrough 310D are coupled to summing device 315A and the outputs ofupconversion elements 310X through 310 n are coupled to summing device315B. The output from summing device 315A is then coupled to the inputof upsample element 320A, while the output from summing device 315B iscoupled to upsample element 320B. The outputs of the second set ofupsample elements—upsample elements 320A and 320B—are then coupled tothe inputs of upconversion elements 330A and 330B. The outputs fromupconversion elements 330A and 330B are coupled to summing device 325.Summing device 325 combines these signals to output an upconverteddigital signal. The output of summing device 325 is then coupled to theinput of DAC system 130.

The embodiment depicted in FIG. 3 provides a tree structure of upsampleand upconversion elements in which two sets of upsample and upconversionelements are used. The description of this embodiment is illustrative,and not intended to limit the invention to a tree structure having onlytwo sets of upsample and upconversion elements. Rather, any number ofsets of upsample and upconversion elements within the tree structure canbe used. The number of sets to be used will be a tradeoff betweenreducing the complexity of individual upsample and upconversion elementsby having a greater number of upsample and upconversion elements, andthe complexity of having an increasing number of upsample andupconversion elements, and summing devices. The number of basebandsignals being converted will factor into the number of sets within atree structure to be used. Based on the teachings herein, individualsskilled in the art can select the appropriate number of sets of upsampleand upconversion elements based on their particular application.

Alternative embodiments of DAC system 130 can also be used within theinvention. In one embodiment, a single digital to analog converter canbe used within DAC system 120. Alternatively, a series of digital toanalog converters can be used. FIG. 4A illustrates the embodiment inwhich a single digital to analog converter 405 is used. In thisembodiment, the output from digital frequency adjustment system 120 iscoupled to the input of digital to analog converter 405. The output ofdigital to analog converter 405 is then provided for distributionthrough a cable television network.

FIG. 4B illustrates an embodiment in which multiple digital to analogconverters are used. In this case, DACs 415, 420, 425 and 430 are used.In the embodiment depicted in FIG. 4B, DAC 415 processes signal band 1,DAC 420 processes signal band 2, DAC 425 processes signal band 3, andDAC 425 processes signal band 4. The outputs of each of DACs 415, 420,425 and 430 are then coupled to the inputs of filters 432, 434, 436 and438. Filters 432, 434, 436 and 438 will be a combination of lowpass,bandpass, and high-pass filters depending on the particular frequency tobe processed. The use of the filters reduces the complexity of thedigital to analog converters. In other embodiments, filters may not beused. The filter outputs are combined by summing device 440 to generatean output signal. By using parallel DACs, the resolution requirement ofan individual DAC is reduced. Specifically, for each factor of fourincrease in the number of DACs, one less bit of resolution is necessary.Thus, while additional hardware is needed, the complexity of thathardware is reduced. The number of parallel DACs may range from 2 to thenumber of bands within the baseband television signal.

FIG. 5A is a diagram of digital hybrid headend upconversion system 500,according to an embodiment of the invention. As in the case of digitalheadend upconversion system 100, digital hybrid headend upconversionsystem 500 converts digital baseband television signals to an analogsignal in which the digital baseband television signals have beenupconverted in frequency to the desired RF frequencies to create amulti-channel RF spectrum. This output, or multi-channel RF spectrum,can then be distributed over a cable television distribution system toindividual cable subscribers.

Digital hybrid headend upconversion system 500 includes demultiplexer505, digital frequency adjustment system 515 and analog frequencyadjustment system 510. Digital hybrid headend upconversion system 500represents a hybrid system in which a portion of the frequencyadjustment occurs within the digital domain and a portion occurs withinthe analog domain.

An input signal is provided to digital hybrid headend upconversionsystem 500 over connection 502, and an output signal is transmitted fromdigital headend upconversion system over connection 504. Digital hybridheadend upconversion system 500 can be used within a cable televisionheadend system. When used within a cable television headend system,inputs into demultiplexer 110 are multiple baseband television channelsin a digital format. The outputs of demultiplexer 110 are coupled to theinput of digital frequency adjustment system 515. The outputs of digitalfrequency adjustment system 515 are coupled to the inputs of analogfrequency adjustment system 510, and the outputs of analog frequencyadjustment system 510 are transmitted over connection 504. In analternate embodiment of digital hybrid headend upconversion system 500,demultiplexed signals are provided to the system and demultiplexer 505is not required.

Digital frequency adjustment system 515 operates under the sameprinciples as described with respect to frequency adjustment system 120with either a single set of upsample and upconversion elements ormultiple sets of upsample and upconversion elements in a tree structure.The differences between digital frequency adjustment system 515 anddigital frequency adjustment system 120 are that (1) digital frequencyadjustment system 515 will not adjust the channel frequencies to thefinal desired channel frequencies and (2) digital frequency adjustmentsystem 515 can provide multiple outputs.

Thus, for example, digital frequency adjustment system 515 can be thesame as digital frequency adjustment system 120 as depicted in FIG. 3,except that summing device 325 would not be used, and the outputs fromupconversion elements 330A and 330B would be coupled to the inputs ofanalog frequency adjustment system 510. Digital frequency adjustmentsystem 515 can have one set of upsample and upconversion elements, ormultiple sets. Additionally, whereas the upsample and upconversionelements in FIG. 3 were selected to upconvert the channel frequencies tothe desired level for distribution within a cable network, the upsampleand upconversion elements used within digital frequency adjustmentsystem 515 can be selected to upconvert the signals to two-thirds (orsome other fraction) of the final desired frequencies. The decision onhow much frequency upconverting will be done by each system is a designdecision based on the particular application, and a cost-benefitanalysis of using upsample and upconversion elements versus usingdigital to analog converters within analog frequency adjustment system510.

FIG. 5B is a diagram of an analog frequency adjustment system 510,according to an embodiment of the invention. Analog frequency adjustmentsystem 510 includes DACs 520 and 525; band pass filters (BPF) 530, 535,550 and 555; mixers 540, 545, 560 and 565; low pass filters (LPF) 570and 575; and summing device 580. In this embodiment, two upconversionprocessing paths are formed for a band 1 and a band 2 of the inputsignal. Band 1 and band 2 represent non-overlapping spectrum bands(e.g., band 1 could be one half of the cable television channels andband 2 could be the other half) of the input signal. The band 1upconversion processing path includes DAC 520, BPF 530, mixer 540, BPF550, mixer 560 and LPF 570. Similarly, band 2 upconversion processingpath includes DAC 525, BPF 535, mixer 545, BPF 555, mixer 565 and LPF575. The processing of these two bands is the same, except for thecenter frequency to which each of band 1 and band 2 will be upconverted.

Along the band 1 upconversion processing path, the signal from whichband 1 is to be upconverted is input into DAC 520. The output of DAC 520is coupled to the input of BPF 530. The output of BPF 530 is band 1upconverted to a center frequency of f₀ to provide a set of firstintermediate signals. The output of BPF 530 is coupled to the input ofmixer 540, which has a frequency of f₁ to provide a set of secondintermediate signals. The output of mixer 540 is coupled to the input ofBPF 550. The output of BPF 550 is the band 1 signal upconverted to acenter frequency of f₁+f₀ to produce a set of third intermediatesignals. The output of BPF 550 is coupled to the input of mixer 560,which has a frequency of f₃. The output of mixer 560 is coupled to LPF570. The output of LPF 570 is the band 1 signal converted to a frequencyof f₁+f₀−f₃=f_(a).

Similarly, along the band 2 upconversion processing path, the signalfrom which band 2 is to be upconverted is input into DAC 525. The outputof DAC 525 is coupled to the input of BPF 535. The output of BPF 535 isband 2 upconverted to a center frequency of f₀. The output of BPF 535 iscoupled to the input of mixer 545, which has a frequency of f₂. Theoutput of mixer 545 is coupled to the input of BPF 555. The output ofBPF 555 is the band 2 signal upconverted to a center frequency of f₂+f₀.The output of BPF 555 is coupled to the input of mixer 565, which has afrequency of f₃. The output of mixer 565 is coupled to LPF 575. Theoutput of LPF 575 is the band 2 signal converted to a frequency off₂+f₀−f₃=f_(b).

The outputs of the band 1 upconversion processing path and band 2upconversion processing path are coupled to the input of summing device580. Summing device 580 combines the signals from band 1 and band 2upconversion processing path to produce an output signal that consistsof the combination of the band 1 signal with a center frequency of f_(a)and the band 2 signal with a center frequency of f_(b).

FIG. 6 is a diagram of an upconversion element 210, according to anembodiment of the invention. Upconversion element 210 consists of acomplex mixer 610 and a digital synthesizer 620. Digital synthesizer 620is coupled to complex mixer 610, such that when an input signal isreceived by complex mixer 610 the frequency can be upconverted using thefrequency provided by digital synthesizer 620. The upconverted signal isthen output from complex mixer 610. In some cases, upconversion element210 can have a transfer function of 1, that is, the frequency of thesignal output is the same as the frequency of the signal input.

FIG. 7 is a method 700 for digital upconversion of baseband televisionsignals, according to an embodiment of the invention. Method 700 beginsin step 710. In step 710, a digital baseband television signal isreceived. In step 720, the received digital baseband television signalis demultiplexed into multiple channels or bands. In step 730, thedemuxed signals are upsampled in frequency. In step 740, the demuxedsignals that have been upsampled are then upconverted. In step 750, thesignals produced in step 740 are summed together to create a singleupconverted digital signal. In step 760, the digital upconverted signalis converted to an analog signal that can be transmitted over a cabledistribution network to individual subscribers. In step 770, method 700ends.

In an alternative embodiment, steps 730, 740, and 750 can serially berepeated multiple times. When they are repeated the frequency will beadjusted only a portion of the desired adjustment on each repeated cycleof these three steps. If these steps are repeated, in step 750, theupsampled and upconverted signals are combined together to produce twoor more composite signals until these series of steps are repeated forthe last time. The last time the steps are repeated, step 750 shouldproduce a single combined single. In step 760 this signal would then beconverted to an analog signal.

FIG. 8 is a method 800 for digital upconversion of baseband televisionsignals, according to an embodiment of the invention. Method 800 beginsin step 810. In step 810, a digital baseband television signal isreceived. In step 815, the received digital baseband television signalis demultiplexed. In step 820, the demuxed signals are upsampled infrequency. In step 825, the demuxed signals that have been upsampled arethen upconverted. In step 830, the signals produced in step 825 aresummed together to create at least two bands containing upconverteddigital signals. In step 835, the bands containing upconverted digitalsignals are converted to analog signals. In step 840, the analog signalsare upconverted in frequency within the analog domain. In step 845, theupconverted analog signals are filtered to extract the desired frequencybands. Steps 840 and 845 can be repeated to upconvert the frequency inmultiple steps, instead of using a single upconversion. In step 850, theextracted frequency bands are combined to create an analog signal fortransmission within a cable television system. In step 870, method 800ends.

In an alternative embodiment, steps 820, 825 and 830 can serially berepeated multiple times. When they are repeated the frequency will beadjusted only a portion of the desired adjustment on each repeated cycleof these three steps. If these steps are repeated, in step 830, theupsampled and upconverted signals are combined together to produce twoor more composite signals. In step 840 outputs produced in step 830would be converted to analog signals.

Exemplary embodiments of digital headend conversion systems and methodsthat can be used to upconvert the frequency of a received digitaltelevision baseband signal to produce an RF multi-channel televisionspectrum for distribution. The present invention is not limited to theseexamples. These examples are presented herein for purposes ofillustration, and not limitation. Alternatives (including equivalents,extensions, variations, deviations, etc., of those described herein)will be apparent to persons skilled in the relevant art(s) based on theteachings contained herein. Such alternatives fall within the scope andspirit of the present invention.

1. A digital headend upconversion system, comprising: (a) means forupward shifting channel frequencies of a set of digital basebandtelevision signals to produce a set of digital upconverted signals; and(b) means for converting the digital up-converted signals to analogsignals.
 2. The digital headend upconversion system of claim 1, furthercomprising means for demultiplexing the digital baseband televisionsignal, wherein the output of said demultiplexing means is coupled tothe input of said upward shifting channel frequency means.
 3. Thedigital headend upconversion system of claim 1, further comprising meansfor receiving the digital baseband television signal, wherein the outputof said receiving means is coupled to the input of said demultiplexingmeans.
 4. The digital headend upconversion system of claim 1, whereinsaid upward shifting means further comprises: (i) means for upsampling aplurality of channel frequencies of a set of digital baseband televisionsignals; (ii) means for frequency upconverting outputs from saidupsampling means; and (iii) means for summing outputs of said frequencyupconverting means to produce an output signal.
 5. The digital headendupconversion system of claim 1, wherein said frequency adjustment systemcomprises: (i) first means for upsampling a plurality of channelfrequencies of a set of digital baseband television signals; (ii) firstmeans for frequency upconverting outputs from said upsampling means;(iii) first means for summing outputs of said frequency upconvertingmeans to produce a first set of output signals; (iv) second means forupsampling a plurality of channel frequencies of a set of digitaltelevision signals contained within the first set of output signals; (v)second means for frequency upconverting outputs from said secondupsampling means.
 6. The digital headend conversion system of claim 5,further comprising a final means for summing outputs of said secondmeans of frequency upconverting outputs to produce a single outputsignal.
 7. The digital headend conversion system of claim 5, furthercomprising a second means for summing outputs of said second means offrequency upconverting outputs to produce multiple output signals.
 8. Adigital hybrid headend upconversion system, comprising: (a) means forupward shifting channel frequencies of a set of digital basebandtelevision signals to produce a set of digital upconverted signals; and(b) means for converting the digital upconverted signals to analogsignals; and (c) means for analog upward shifting the channelfrequencies of the upconverted analog signals generated by saidconverting means.
 9. The hybrid digital headend upconversion system ofclaim 8, further comprising means for demultiplexing the digitalbaseband television signal, wherein the output of said demultiplexingmeans is coupled to the input of said upward shifting channel frequencymeans.
 10. The hybrid digital headend upconversion system of claim 8,further comprising means for receiving the digital baseband televisionsignal, wherein the output of said receiving means is coupled to theinput of said demultiplexing means.
 11. The digital hybrid headendupconversion system of claim 8, wherein said upward shifting meansfurther comprises: (i) means for upsampling a plurality of channelfrequencies of a set of digital baseband television signals; (ii) meansfor frequency upconverting outputs from a said upsampling means; and(iii) means for summing outputs of said frequency upconverting means toproduce an output signal.
 12. The digital hybrid headend upconversionsystem of claim 8, wherein said analog upward shifting means furthercomprises: (i) first means for passing a portion of a signal within aparticular frequency range for a plurality of signals, wherein saidfirst passing means is coupled to the output of said converting means;(ii) first means for mixing a second frequency with a first frequencyfor a plurality of signals, wherein said first mixing means is coupledto the output of said first passing means; (iii) second means forpassing a portion of signal within a particular frequency range for aplurality of signals, wherein said second passing means is coupled tothe output of said converting means; (iv) second means for mixing asecond frequency with a first frequency for a plurality of signals,wherein said second mixing means is coupled to the output of said secondpassing means; and (v) third means for passing a portion of a signalwithin a particular frequency range to produce an output signal, whereinsaid third passing means is coupled to the output of said second mixingmeans.